In an interview with NowGamer, chief technical officer at Metro: Last Light developer 4A Oles Shishkovtsov said probably the most disparaging thing you can say about a brand new console: it has no power. When asked if the new game would be on the Wii U, Shishkovtsov said that it was not possible because Wii U's processor was "horrible and slow." Not exactly what the folks at Nintendo headquarters want a developer to say...
THQ spokesperson Huw Beynon explained Shishkovtsov's claims in friendlier terms (they are publishing the game):
"We had an early look at it, [and] we thought we could probably do it," he said. "But in terms of the impact we would make on the overall quality of the game - potentially to its detriment - we just figured it wasn't worth pursuing at this time. It's something we might return to. I really couldn't make any promises, though."
The processing speed of the Wii U's processor is unknown so it is not possible to verify the claim, and plenty of other studios such as Assassin's Creed 3 developer Ubisoft Montpellier and Black Ops 2 developer Treyarch have created games for the platform.